AURORA, Colo. — Anticipation built in the packed, darkened movie theater. Life and its cares began to recede.

Then, just after midnight on Friday, fantasy became nightmare, and a place of escape became a trap, when a man strode to the front in a multiplex near Denver and opened fire. At least 12 people were killed and 58 wounded, with witnesses describing a scene of claustrophobia, panic and blood. Minutes later, the police arrested James Holmes, 24, in the theater’s parking lot.

“It was just chaos. You started hearing screaming. You looked up and people were falling. It was like a dream,” said Jamie Rohrs, 25, who was there with his fiancée, cradling his 4-month-old son, Ethan, in his arms as the movie began. It was the midnight premiere of “The Dark Knight Rises,” the latest Batman sequel, at the Century 16 multiplex in Aurora, about 10 miles from downtown Denver.

Mr. Rohrs jumped between the seats for cover, still holding the baby. He stumbled and crawled trying to figure out what to do, clutching his son to his chest as he went. “Do I run out the door? Is he going to shoot the baby? What am I to do?” Mr. Rohrs said, his voice quavering. But he, his fiancée and the baby eventually made it out.

And so once again, with a squeeze of a trigger, just 20 miles from Columbine High School, scene of the 1999 student massacre, the nation was plunged into another debate about guns and violence.

Mayor Michael R. Bloomberg of New York, who has waged a national campaign for stricter gun laws, offered a political challenge. “Maybe it’s time that the two people who want to be president of the United States stand up and tell us what they are going to do about it,” Mr. Bloomberg said during his weekly radio program, “because this is obviously a problem across the country.”

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James Holmes, 24, is seen in this undated handout picture released by the University of Colorado.Credit
Reuters

Luke O’Dell of the Rocky Mountain Gun Owners, a Colorado group on the other side of the debate over gun control, took a nearly opposite view. “Potentially, if there had been a law-abiding citizen who had been able to carry in the theater, it’s possible the death toll would have been less.”

Some survivors thought at first they were witnessing a promotional stunt. The gunman, wearing what Aurora Police Department officials described as nearly head-to-toe “ballistic gear,” including a throat protector and leggings, plus a gas mask and a long black coat, came in through a parking lot exit door near the screen of Theater 9.

“He walked in so casually,” said a witness, Jordan Crofter, 19, a Batman fan who had gone with a group of friends and had a seat in the front row. The gunman, still perhaps regarded by some as a performer, then released two devices down the theater aisles emitting what the police said was smoke or some sort of irritant.

Witnesses told the police that Mr. Holmes said something to the effect of “I am the Joker,” according to a federal law enforcement official, and that his hair had been dyed or he was wearing a wig. Then, as people began to rise from their seats in confusion or anxiety, he began to shoot. The gunman paused at least once, several witnesses said, perhaps to reload, and continued firing.

Mr. Holmes was detained by the police soon afterward, standing by his white Hyundai. He was identified by the authorities as a former Ph.D. student at the University of Colorado in Denver, and an honors graduate in neuroscience from the University of California, Riverside. He had in the car an AR-15 assault rifle, a Remington 12-gauge shotgun, and a .40 caliber Glock handgun, said Chief Dan Oates of the Aurora police, and all three were believed to have been used inside the theater. Another Glock .40 caliber handgun was recovered inside the theater. Chief Oates said that “many, many” rounds were fired, but that there was no count so far.

In the last 60 days Mr. Holmes had purchased four guns at local gun shops, Chief Oates said. And through the Internet, he bought more than 6,000 rounds of ammunition: more than 3,000 rounds for the assault rifle, 3,000 rounds of .40 caliber ammunition for the two Glocks, and 300 rounds for the 12-gauge shotgun. The guns were all bought legally, a federal law enforcement official said.

Mr. Holmes also purchased online multiple magazines for the assault rifle, including one 100-round drum magazine. “With that drum magazine, he could have gotten off 50, 60 rounds, even if it was semiautomatic, within one minute,” Chief Oates said.

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A gunman wearing a gas mask opened fire shortly after midnight Friday at a movie in the Century 16 multiplex in Aurora, Colo.Credit
David Zalubowski/Associated Press

The Federal Bureau of Alcohol, Tobacco and Firearms has given information on where and when the guns were purchased to the police in Aurora.

After the chaos — the evacuation, the rush of 911 calls beginning at 12:39 a.m., the mass casualties rushed to area hospitals, the images that witnesses described to authorities of dead or dying friends or loved ones left inside — the emerging details got more mysterious and in some ways more horrifying still.

“Our investigation determined his apartment is booby trapped with various incendiary and chemical devices and apparent trip wires,” Chief Oates said around midday Friday. “We have an active and difficult scene. It may be resolved in hours or days. We simply don’t know how we’re going to handle that.” Federal law enforcement officials said that the apartment had been so extensively booby-trapped that the devices could not be safely defused and that a robot would be sent to trigger them. Late Friday, the dozens of residents in five apartment buildings in the area remained evacuated.

“It is a very vexing problem how to enter that apartment safely,” Chief Oates said. “Personally I’ve never seen anything like what the pictures show us is in there. I’m a layman when it comes to bomb stuff. I see an awful lot of wires. Trip wires. Jars full of ammunition. Jars full of liquid. Some things that look like mortar rounds. We have a lot of challenges to get in there safely.”

In the suspect’s neighborhood of low-slung, red-brick apartment buildings and dusty lots, not far from Children’s Hospital Colorado, where some of the victims were taken, neighbors shook their heads in disbelief as the police and bomb squad vehicles cordoned off the area.

“I think this is a lot like Columbine,” said Jennifer Evans, who lives near Mr. Holmes’s apartment. “This is crazy.”

At one point, as shirtless children played in an overgrown front yard and day laborers stared in disbelief, firefighters in a hook and ladder truck smashed a window in the third story of the building where the police said Mr. Holmes resided.

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Jessica Ghawi, a victim, was a reporter and active on Twitter.Credit
Agence France-Presse — Getty Images

Whether a violent movie had inspired violence in real life, or was merely the coincidental setting, police authorities in some other cities, including New York, said they were taking no chances and had increased security at theaters showing the movie.

In Paris, Warner Bros. canceled a red-carpet premiere of “The Dark Knight Rises,” along with other promotional events. On Friday afternoon workers removed a giant Batman mask that had been mounted on the front of a theater along the Champs-Élysées, where the screening was to take place.

Warner Bros., which is owned by Time Warner, released a statement Friday, saying that the company and the filmmakers were “deeply saddened” and “extend our sincere sympathies to the families and loved ones of the victims at this tragic time.” Several broadcast networks and cable channels stopped running commercials on Friday for “The Dark Knight Rises.”

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The shooting inevitably stirred memories of Columbine High School and the murders there in April 1999, when two heavily armed students stalked through the hallways, killing 12 students and a teacher as they went, before shooting themselves.

And the psychological echo and the similar feel of the two massacres was palpable: Theater 9 was a place of seeming safety, if not sanctuary, not unlike Columbine’s library, where some of the killings occurred. Both were ordinary settings that became death traps.

Mr. Holmes seemed ordinary too, for the most part, said Billy Kromka, a premed student at the University of Colorado, Boulder, who worked with Mr. Holmes for three months last summer as a research assistant.

“There was no way I thought he could have the capacity to commit an atrocity like this,” said Mr. Kromka, who is from Aurora. Mr. Kromka said that Mr. Holmes’s “disposition was a little off” and that he could be socially awkward, one of the quieter people in the lab. He spent much of his time immersed in his computer, often participating in role-playing online games.

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Colorado Movie Theater Shooting

A shooting at a movie theater near Denver early Friday killed at least 12 and injured dozens.

His criminal history with the Aurora Police Department consisted before Friday of one traffic summons, for speeding, in October.

A spokeswoman for the University of Colorado Denver-Anschutz Medical Campus, Jackie Brinkman, said Mr. Holmes was in the process of dropping out of school because of academic problems. She said the university was unaware of any incidents with campus police or disciplinary problems involving Mr. Holmes while he was enrolled.

Mr. Holmes grew up in a quiet, middle-class community at the eastern edge of San Diego, where his parents still live. Two-story, Spanish-style tract homes line both sides of the street, all white stucco with red tile roofs and well-kept lawns. His mother, Arlene Rosemary Holmes, is a registered nurse.

One neighbor, Margie Aguilar, said she knew the Holmes family, who she said had lived in the area for at least 10 years. Her son was a little younger than Mr. Holmes and attended the same high school, Westview, which is just up the street.

One of the victims, Jessica Ghawi, was a 25-year-old college student and sports broadcaster active on Twitter under the name Jessica Redfield — a tribute to her red hair. She posted early Friday that she was at the movie screening after convincing a friend to go with her.

Her brother, Jordan Ghawi, said in a blog post on Friday that when the gunfire began, Jessica took two rounds. “My sister took one round followed by an additional round which appeared to strike her in the head,” he wrote.

The shots in Toronto rang out in the food court striking seven people about 6:23 p.m., she recalled. “It’s hard for me to wrap my mind around how a weird feeling saved me from being in the middle of a deadly shooting,” she wrote on her blog.

A version of this article appears in print on July 21, 2012, on Page A1 of the New York edition with the headline: Gunman Kills 12 At Colorado Theater; Scores Are Wounded, Reviving Debate. Order Reprints|Today's Paper|Subscribe